1. Field of the Invention
The present invention relates to a rubber composition and to a pneumatic tire using the rubber composition, and more particularly, to a rubber composition and to a pneumatic tire using the rubber composition which has excellent braking ability on a wet road surface (hereinafter referred to as "wet skid resistance"), excellent rolling resistance, and excellent abrasion resistance.
2. Description of the Related Art
Recently, because of social requirements to save energy and to save resources, particularly to cut down fuel consumption of automobiles, a decrease in the rolling resistance of tires is also required. For example, Japanese Patent Application Publication No. Sho-62-10530 discloses a technology in which wet skid resistance, rolling resistance, and abrasion resistance of a pneumatic tire are improved by using a tin coupling polymer having a specific microstructure for tread rubber of a tire.
In this improved mechanism, a bonding portion between tin and carbon is cut during mixing of a rubber, the generated polymer radicals react with a quinone group on the surface of carbon black, and dispersion of the carbon black into the rubber is improved. In order to carry out the reaction efficiently, it is preferable that the mixing temperature of a rubber is high.
Further, Japanese Patent Application Laid-Open (hereinafter, "JP-A") No. Hei-3-252433 discloses a technology in which a polymer terminal is modified by an alkoxy group and the modified polymer is caused to react with silica. In order to cause to react the polymer terminal alkoxy group react with the silica, it is again preferable that the mixing temperature of a rubber is high.
As mentioned above, in any of the solution polymerization coupling polymers, it is necessary that the mixing temperature of a rubber is high for bringing out the characteristics sufficiently.
On the other hand, it has been reported that silica is used for the tread rubber composition of a tire in order to achieve low fuel consumption and good wet skid resistance of a tire. For example, in JP-A No. Hei-3-252431, JP-A No. Hei-4-224840, JP-A No. Hei-5-271477, JP-A No. Hei-5-51484, JP-A No. Hei-7-1908, and JP-A No. Hei-7-48476, silica is used for a tread rubber so as to achieve low fuel consumption, good wet skid resistance, and good abrasion resistance.
However, in a case in which silica is used into rubber, silica particles tend to cohere together due to hydrogen bonding of silanol groups which are functional groups on the surfaces of the silica particles. For improving the dispersion of silica particles into rubber, the mixing time must be increased. Moreover, the surfaces of the silica particles are acidic. Therefore, there are problems in that basic substances used as vulcanization accelerators are absorbed such that vulcanization is not carried out sufficiently, and a sufficient modulus of elasticity is not obtained.
In order to solve these problems, various types of silane coupling agents have been developed. For example, use of a silane coupling agent as a reinforcing material is described in Japanese Patent Application Publication No. Sho-50-29741. However, the use of a silane coupling agent as a reinforcing material is still insufficient for improving fracture properties, workability, and processability of a rubber composition to high standards. Rubber compositions in which a combination of silica and silane coupling agent is used as a reinforcing material are described in Japanese Patent Application Publication No. Sho-51-20208 and others. However, this method of using a combination of silica and silane coupling agent as a reinforcing material has a drawback in that flow of the uncured rubber composition is markedly inferior and workability and processability deteriorate, although reinforcement of the rubber composition can be remarkably improved and fracture properties are improved.
The drawbacks of the conventional technologies in which silane coupling agents are used arise due to the following mechanism. When the mixing temperature of rubber is low, the silanol group on the surface of the silica does not react sufficiently with the silane coupling agent, and as a result, the sufficient reinforcing effect is not obtained. Dispersion of the silica into the rubber is also inferior, and this causes deterioration of the low heat buildup property which is the strong point of a rubber composition containing silica. Moreover, some of the alcohol formed by the reaction of the silanol group on the surface of the silica and the silane coupling agent does not vaporize completely during mixing because of the low mixing temperature, and the residual alcohol in the rubber vaporizes during an extrusion process so as to form blisters.
On the other hand, when mixing is conducted at high temperatures of 150 .degree. C. or more, the silanol group on the surface of the silica and the silane coupling agent sufficiently react with each other, and as a result, the reinforcing property is improved. Because dispersion of the silica into the rubber is also improved, a rubber composition having a low heat buildup property is obtained, and the formation of blisters in an extrusion process is suppressed. However, in this temperature range, gelation of the polymer caused by the silane coupling agent takes place simultaneously, and the Mooney viscosity is markedly increased. Thus, processing in later stages becomes impossible in actuality.
As mentioned above, as long as a conventional silane coupling agent is used, a sufficient mixing cannot be effected at a high temperature. Accordingly, when carbon black and silica are combined, it is impossible to bring out sufficiently the characteristics of a so-called solution polymerization coupling polymer which is prepared by using an organic lithium initiator.